1. Field of the Invention
The present invention relates to a vibration actuator for effecting paging which is incorporated in a portable telephone or the like to notify a user of the arrival of a received call not only by sound but also by vibration. More particularly, the present invention relates to the vibration actuator for paging suited for achieving a smaller, lighter design.
2. Description of the Related Art
Conventional vibration actuators for paging are also called vibration motors or vibration-generating actuators. They are required to be compact, thin and inexpensive, and also to be capable of generating vibration at low power consumption. They are intended, however, to generate only vibration. Therefore, they cannot be used for making a voice call or issuing conversational voice. Hence, at least two component units are necessary for obtaining the information on incoming calls and for generating voice signals. Further, the pager vibrating motors which are extensively used consume much starting power for rotating a relatively large mass. They are also disadvantageous in that they have many components because of the rotating design, and provide unsatisfactory reliability or accuracy control. The conventional pager vibrating motors have a further disadvantage, namely, they include current switching brushes because they use direct current, so that they may generate large electromagnetic noises or incur a malfunction at the time of rotation, and they also have a limitation in accomplishing a smaller and flatter design.
FIG. 1 shows a pager vibrating motor which has been most commonly used hitherto. A counterweight 3 is rotated via a shaft 2 driven by a driving motor 1 composed of a cylindrical coreless rotor. The counterweight swings to generate vibration. Naturally, the pager vibrating motor is not capable of generating sound other than the vibration. The driving motor 1 is constituted by a permanent magnet having a curved surface and the cylindrical coreless rotor; and a plurality of magnetic poles to provide rotational driving force. This places restrictions on accuracy control and manufacturing cost in achieving a smaller diameter of the driving motor 1.
FIG. 2 shows the cylindrical pager vibrating motor in a vibrating state. As the driving motor 1 runs, the counterweight 3 swings around a rotational center 4. The vibration is generated in many directions, and therefore, the vibration in a particular direction may not be effectively transferred outside, depending on how the vibration motor for a pager is fixed. Further, a driving force is mandatory because the swinging moment is proportional to the square of the rotational speed of the driving motor 1, thus limiting the efforts in saving power.
FIG. 3 is a perspective view illustrative of the interior of a flat type pager vibrating motor 5 constructed by a conventional flat coreless rotor. A rotary shaft 8 is provided with a disc-shaped winding coil 6 with an eccentric center of gravity to generate a rotational driving force between the winding coil 6 and a sheet-like permanent magnet 7. Driving current is supplied through a brush 9. Unlike the cylindrical motor, the pager vibrating motor 5 makes use of the winding coil 6 with the eccentric center of gravity in place of the counterweight. The winding coil produces vibration when it revolves. Obviously, this pager vibrating motor 5 also cannot be used for generating the voice.
FIG. 4 shows the most effective vibration of the flat type pager vibrating motor 5. The rotations in the axial direction relative to a vibration central axis 10 are indicated by reference numerals 5, 11 and 12 assigned to the main body of the pager vibrating motor 5. There are thickness vibration in the axial direction and diametral vibration at right angles to the axis 10. Frequently, however, the vibrations may hardly contribute to the vibration to be transferred outside, depending on the way the flat type pager vibrating motor 5 is fixed. This means that the driving current applied to the winding coil 6 is not effectively used as the energy for transferring the vibration outside.
Thus, the conventional pager vibrating actuators have not been able to generate sound and voice although they are able to generate vibration. Furthermore, the conventional pager vibrating actuators have not necessarily been able to reduce the required starting power, they have hardly permitted a smaller overall size, and some of them have been prone to rotational malfunction, producing large electromagnetic noises.